The present invention relates generally to methods for generating highly n-doped strained Ge films as well as semiconductor structures that include such films.
Germanium has emerged as a promising material platform for the next generation of silicon CMOS compatible devices. Its high electron and hole mobilities, a direct transition at 1.55 μm, and the possibility to grow it directly on a silicon substrate makes germanium (Ge) especially desirable for silicon integrated electronic and photonic applications. Ge on silicon is becoming a mainstream material, already used in commercial infrared photodetectors and high speed MOSFETs.
Despite this success, the challenge of obtaining high active n+Ge doping has been a persistent bottleneck to further development of Ge devices. High active n+ carrier concentrations are desirable for forming Ohmic contact in fabrication of low parasitic resistance n-MOSFETs and for facilitating population inversion and lasing emission near 1.55 μm. In many such applications, not only a high active n+Ge doping but also a high tensile strain is also desirable.
Conventional doping methods like in-situ, gas-phase doping, or conventional ion implantation followed by a standard thermal annealing, are unable to achieve an active donor concentration of greater than about 5×1019 cm−3, regardless of the chemical concentration of donors. Some studies suggest that this low carrier activation may be due to the formation of negatively charged donor-vacancy (DV) clusters that compensate acceptors.
A number of approaches have been proposed to manage the vacancies and DV pairs that may be the cause of low donor activation, including multiple implantations, irradiation to introduce interstitials and co-doping. But such approaches have not been fruitful in generating high active n+ carrier concentrations in Ge.
Accordingly, there is a need for methods for fabricating highly n-doped strained Ge films, and for semiconductor structures including such films.